This invention is directed to integrated circuits, and to improvements in the interconnections used therein.
Most modern electronic systems include one or more integrated circuit (IC) chips bonded to a suitable ceramic substrate, which in turn is connected to an electronic package such as a module, card, board, or gate.
In many of these electronic systems, solder bump interconnections are used in order to eliminate the expense, unreliability, and low productivity, of wirebonding of chips to their substrate. Thus, the so-called Controlled Collapse Chip Connection (C4), for example, utilizes solder bumps deposited on solder wettable metal terminals on the IC chip, and matching footprints of solder wettable terminals on the substrate.
This upside-down chip or flip chip is aligned to the substrate, and all of the joints are made simultaneously by reflowing the solder. Flip chip joining is opposite to traditional back-side-down methods of bonding, in which the active side of the chip, i.e. facing up, is wirebonded. In the flip chip, the back of the chip is free of mechanically or electrically delicate surface features.
While the flip chip interconnection method has been largely successful in various applications, it suffers from certain disadvantages, among which are that it is susceptible to mismatches between the coefficients of thermal expansion of the chip and the substrate. Thus, mismatches between these materials results in high shear stresses in the solder joints, which compromises the reliability of the flip chip interconnection.
What we propose as a solution is a new conductive adhesive containing randomly dispersed populations of conductive particles. The particles can be distributed so as to provide electrical conductivity through only the thickness of the adhesive, or the particles can be distributed so as to provide electrical conductivity through the bulk of the adhesive in all directions. Our adhesive possesses the advantages of being repairable, reworkable, and it can be entirely removed from a bonded device with heat and/or solvent. These features allow for devices to be temporarily positioned, tested, removed, and repositioned permanently, without damage to the device. During the process, if one or more of the chips are found to be defective, it can be removed and replaced with ease.